java.util
Interface Map

An object that maps keys to values. A map cannot contain duplicate keys;
each key can map to at most one value.

This interface takes the place of the Dictionary class, which
was a totally abstract class rather than an interface.

The Map interface provides three collection views, which
allow a map's contents to be viewed as a set of keys, collection of values,
or set of key-value mappings. The order of a map is defined as
the order in which the iterators on the map's collection views return their
elements. Some map implementations, like the TreeMap class, make
specific guarantees as to their order; others, like the HashMap
class, do not.

Note: great care must be exercised if mutable objects are used as map
keys. The behavior of a map is not specified if the value of an object is
changed in a manner that affects equals comparisons while the object is a
key in the map. A special case of this prohibition is that it is not
permissible for a map to contain itself as a key. While it is permissible
for a map to contain itself as a value, extreme caution is advised: the
equals and hashCode methods are no longer well defined on a such a map.

All general-purpose map implementation classes should provide two
"standard" constructors: a void (no arguments) constructor which creates an
empty map, and a constructor with a single argument of type Map,
which creates a new map with the same key-value mappings as its argument.
In effect, the latter constructor allows the user to copy any map,
producing an equivalent map of the desired class. There is no way to
enforce this recommendation (as interfaces cannot contain constructors) but
all of the general-purpose map implementations in the SDK comply.

The "destructive" methods contained in this interface, that is, the
methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException if this map does not support the
operation. If this is the case, these methods may, but are not required
to, throw an UnsupportedOperationException if the invocation would
have no effect on the map. For example, invoking the putAll(Map)
method on an unmodifiable map may, but is not required to, throw the
exception if the map whose mappings are to be "superimposed" is empty.

Some map implementations have restrictions on the keys and values they
may contain. For example, some implementations prohibit null keys and
values, and some have restrictions on the types of their keys. Attempting
to insert an ineligible key or value throws an unchecked exception,
typically NullPointerException or ClassCastException.
Attempting to query the presence of an ineligible key or value may throw an
exception, or it may simply return false; some implementations will exhibit
the former behavior and some will exhibit the latter. More generally,
attempting an operation on an ineligible key or value whose completion
would not result in the insertion of an ineligible element into the map may
throw an exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
interface.

isEmpty

containsKey

Returns true if this map contains a mapping for the specified
key. More formally, returns true if and only if
this map contains at a mapping for a key k such that
(key==null ? k==null : key.equals(k)). (There can be
at most one such mapping.)

containsValue

Returns true if this map maps one or more keys to the
specified value. More formally, returns true if and only if
this map contains at least one mapping to a value v such that
(value==null ? v==null : value.equals(v)). This operation
will probably require time linear in the map size for most
implementations of the Map interface.

get

Returns the value to which this map maps the specified key. Returns
null if the map contains no mapping for this key. A return
value of null does not necessarily indicate that the
map contains no mapping for the key; it's also possible that the map
explicitly maps the key to null. The containsKey
operation may be used to distinguish these two cases.

More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)

Parameters:

key - key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key, or
null if the map contains no mapping for this key.

put

Associates the specified value with the specified key in this map
(optional operation). If the map previously contained a mapping for
this key, the old value is replaced by the specified value. (A map
m is said to contain a mapping for a key k if and only
if m.containsKey(k) would return
true.))

Parameters:

key - key with which the specified value is to be associated.

value - value to be associated with the specified key.

Returns:

previous value associated with specified key, or null
if there was no mapping for key. A null return can
also indicate that the map previously associated null
with the specified key, if the implementation supports
null values.

remove

Removes the mapping for this key from this map if it is present
(optional operation). More formally, if this map contains a mapping
from key k to value v such that
(key==null ? k==null : key.equals(k)), that mapping
is removed. (The map can contain at most one such mapping.)

Returns the value to which the map previously associated the key, or
null if the map contained no mapping for this key. (A
null return can also indicate that the map previously
associated null with the specified key if the implementation
supports null values.) The map will not contain a mapping for
the specified key once the call returns.

Parameters:

key - key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null
if there was no mapping for key.

putAll

Copies all of the mappings from the specified map to this map
(optional operation). The effect of this call is equivalent to that
of calling put(k, v) on this map once
for each mapping from key k to value v in the
specified map. The behavior of this operation is unspecified if the
specified map is modified while the operation is in progress.

clear

keySet

Returns a set view of the keys contained in this map. The set is
backed by the map, so changes to the map are reflected in the set, and
vice-versa. If the map is modified while an iteration over the set is
in progress, the results of the iteration are undefined. The set
supports element removal, which removes the corresponding mapping from
the map, via the Iterator.remove, Set.remove,
removeAllretainAll, and clear operations.
It does not support the add or addAll operations.

Returns:

a set view of the keys contained in this map.

values

Returns a collection view of the values contained in this map. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. If the map is modified while an
iteration over the collection is in progress, the results of the
iteration are undefined. The collection supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll and clear operations.
It does not support the add or addAll operations.

Returns:

a collection view of the values contained in this map.

entrySet

Returns a set view of the mappings contained in this map. Each element
in the returned set is a Map.Entry. The set is backed by the
map, so changes to the map are reflected in the set, and vice-versa.
If the map is modified while an iteration over the set is in progress,
the results of the iteration are undefined. The set supports element
removal, which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove, removeAll,
retainAll and clear operations. It does not support
the add or addAll operations.

Returns:

a set view of the mappings contained in this map.

equals

Compares the specified object with this map for equality. Returns
true if the given object is also a map and the two Maps
represent the same mappings. More formally, two maps t1 and
t2 represent the same mappings if
t1.entrySet().equals(t2.entrySet()). This ensures that the
equals method works properly across different implementations
of the Map interface.

hashCode

public int hashCode()

Returns the hash code value for this map. The hash code of a map
is defined to be the sum of the hashCodes of each entry in the map's
entrySet view. This ensures that t1.equals(t2) implies
that t1.hashCode()==t2.hashCode() for any two maps
t1 and t2, as required by the general
contract of Object.hashCode.

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